This invention relates to decoding of received signals and, more particularly, to decoding quantized and unquantized wanted data symbols from received signal samples.
The code-division multiple access (CDMA) mobile communications system known as IS95 transmits from a base station to different mobile terminals in its coverage area (the downlink) using different 64-bit orthogonal codes. Each such code is of the same length (64 bits) and carries voice or data traffic of approximately the same data rate. Variable-rate orthogonal coding is not used in that system.
A wideband CDMA system known as 3G (third generation) has been standardized in a cooperation between the European Telecommunications Standards Institute (ETSI) and NTT DoCoMo of Japan, and specifies variable-rate orthogonal coding in which signals of higher bitrate can use orthogonal codes of a shorter length to increase the frequency of data symbol transmission while still remaining orthogonal to lower bitrate transmissions using longer orthogonal codes and a lower frequency of data symbol transmission. The shortest orthogonal code presently specified is 4 chips long and the longest is 256 chips long.
Also in IS95, transmissions from mobile terminals to base stations use orthogonal codes to code different data symbol groups from the same mobile terminal but do not use orthogonal codes to distinguish between different mobile terminals. Different mobile terminal transmissions (the uplink) are distinguished by the use of different non-orthogonal, pseudorandom codes.
U.S. Pat. No. 5,151,919 entitled “CDMA Subtractive Demodulation” and U.S. Pat. No. 5,218,619 also entitled “CDMA Subtractive Demodulation” to Applicant describe a CDMA system using orthogonal codes in the manner of the above-described IS95 uplink, in which different signals are successively decoded and subtracted in order from strongest to weakest in order to eliminate interference of the stronger signals upon the weaker signals. The '919 and '619 patents are hereby incorporated by reference herein.
In U.S. Pat. No. 5,572,552 entitled “Method and system for demodulation of downlink CDMA signals”, Dent and Bottomley describe an optimum receiver for receiving CDMA signals at a mobile terminal transmitted from a cellular base station that subtracts non-orthogonal multipath rays in a multipath channel equalizer when own-base interference is dominant. Such a “channel inverse” equalizer is disclosed to be non-optimum in the presence of other-base interference or thermal noise and a hybrid equalizer method is described that lies between the conventional RAKE equalizer method and the channel inverse equalizer method. The '552 patent is hereby incorporated by reference herein.
In U.S. patent application Ser. No. 09/082,722, filed May 21, 1998, entitled “Partially Block-interleaved CDMA Coding and Decoding” to Applicant, methods for transmitting and receiving orthogonally-coded CDMA signals are described such that signals retain their orthogonality for most transmitted data symbols under multipath conditions. The above application is hereby incorporated by reference herein.
In U.S. Patent application Ser. No. 08/898,392, filed Jul. 22, 1997, entitled “Orthogonal Block-Spreading Codes for the Multipath Environment”) further methods are described for compensating for multi-user interference on the residual symbols not retaining their mutual orthogonality by virtue of the Block-interleaving method. This application is hereby incorporated by reference herein.
Also, in U.S. patent application Ser. No. 09/340,907, filed Jun. 28, 1999, entitled “Multi-Carrier Orthogonal Coding”, methods are disclosed for transmitting and receiving CDMA signals that are orthogonally coded over more than one frequency channel. This application is hereby incorporated by reference herein.